Instruments measuring the alcohol content in a breath sample have become a common and effective tool utilized by business human resource personnel, government officials, and law enforcement agencies to evaluate whether a person is under the influence of alcohol. The statutory offense of driving-under-the-influence is defined in most states as operating a vehicle while having a blood alcohol content greater than 0.10 percent. A typical breath alcohol measuring instrument utilizes infrared radiation (IR) to calculate the content of ethyl alcohol (ethanol) in a breath sample, from which blood alcohol content is readily calculable.
Such an instrument utilizing infrared radiation includes a sample cell forming a chamber that receives and retains a breath sample. A person being tested blows a breath sample into the chamber and infrared radiation attenuation at at least two different wavelengths is monitored during the residence time of the breath sample in the chamber. The first wavelength is selected to be attenuated by the ethanol while the second wavelength is not attenuated. The signal representing the second wavelength is compared to a signal representing a baseline value stored in the system's computer, and is used primarily to stabilize the instrument to prevent long term drift due to aging of components, such as the inevitable weakening of the IR source. The amount of the attenuation at the first wavelength is calculated by the computer by comparing the difference between the signals representing the radiation received before the breath sample, to that received when the sample is present in the chamber.
Precise and reliable calibration of infrared breath alcohol measuring instruments is very important because even small errors in the measured attenuation can cause relatively large errors in the alcohol content determined by the computer. To avoid improper arrest and charges, including for driving-under-the-influence, and to maintain the credibility of the instruments and alcohol readings taken thereby, reliable and precise calibration is required.
The most common calibration check technique for infrared breath alcohol measuring instruments is the "wet" method. Briefly in this prior art method, a gas/vapor of known alcohol content is introduced into the sample chamber, and infrared radiation is passed therethrough. The attenuation induced by the vapor is measured and the alcohol content in the vapor is determined by the computer. The determined alcohol content is then compared to the known alcohol content (usually 0.10%) of the calibration gas. In the unlikely event that the determined alcohol content and the known alcohol content differs, appropriate adjustment or other remedial action is taken, and the instrument is tested again until the two readings match.
The "wet" method of checking calibration possesses significant drawbacks that have provided impetus for a search for an improved calibration check means and method. One specific disadvantage is that a sufficient supply of a calibration mixture of water and ethanol must be maintained with the instrument, even in the field applications. Typically this means hauling around a bulky tank of water/ethanol with the instrument. Another disadvantage is that the water/alcohol mixture must be maintained at a suitable, relatively high temperature to form the vapor prior to introduction into the instrument. This adds a need for providing bulky electric heaters, especially in colder climates, as well as limiting use to locations where an electric power source is available.
A relatively recent innovation in calibrating breath alcohol measuring instruments has been the introduction of solid calibration plates to replace the wet method of calibration. A solid calibration plate is placed in the path of infrared radiation and simulates a breath sample of the predefined alcohol content by causing similar attenuation of the radiation. Solid calibration plates disclosed in the prior art include various partially IR transparent substrates allowing the correct amount of infrared radiation to pass therethrough. One example of such a device is a comb mask, as disclosed in U.S. Pat. No. 3,562,524.
These known prior art solid calibration plates, in spite of the decided advantages over the wet method, are not widely in use, mainly because of questionable reliability. Also, some substrates previously proposed for use have been problematic due to lack of durability. This is a particular problem in mobile installations. Fine scratches or smudges on the substrates, whether formed on an interrupted or solid body, significantly affect the attenuation of infrared radiation, making them unreliable for calibration, especially when they are handled by hand. This condition can sometimes be hard to detect, especially by an inexperienced operator. If the defects are not immediately detected, an improper conclusion is often reached that the instrument is in need of adjustment or repair, instead of the calibration plate being defective.
Thus, as demonstrated by these disadvantages, there is a need identified for an easily transported, yet reliable means for checking calibration of a breath alcohol measuring instrument.